The beat of the heart is controlled by the sinoatrial node, a group of conductive cells located in the right atrium near the entrance of the superior vena cava. The depolarization signal generated by the sinoatrial node activates the atrioventricular node. The atrioventricular node briefly delays the propagation of the depolarization signal, allowing the atria to drain, before passing the depolarization signal to the ventricles of the heart. The coordinated contraction of both ventricles drives the flow of blood through the torso of a patient. In certain circumstances, the conduction of the depolarization signal from the atrioventricular node to the left and right ventricles may be interrupted or slowed. This may result in a dyssynchrony in the contraction of the left and right ventricles, and eventually in heart failure or death.
Cardiac Resynchronization Therapy (CRT) may correct the symptoms of electrical dyssynchrony by providing pacing therapy to one or both ventricles or atria, e.g., by providing pacing to encourage earlier activation of the left or right ventricles. By pacing the contraction of the ventricles, the ventricles may be controlled so that the ventricles contract in synchrony. Some patients undergoing CRT have experienced improved ejection fraction, increased exercise capacity, and an improved feeling of well-being.
Providing CRT to a patient may involve determining whether the patient will derive benefit from the CRT prior to implantation of a cardiac rhythm device, determining optimal site for placement of one or more ventricular pacing leads, and programming of device parameters, such as selection of electrodes on multi-polar right or left ventricular leads, as well as selection of the timing of the pacing pulses delivered to the electrodes, such as atrioventricular (A-V) and intra-ventricular (V-V) delays. Assessment of electrical dyssynchrony for these purposes has typically involved assessing QRS duration clinically. Though CRT is recommended typically for patients with wide QRS duration, hemodynamic improvements through CRT have been reported in narrow QRS heart failure patients. Thus, some patients who may benefit from CRT may not be prescribed CRT based on present electrical dyssynchrony evaluation techniques.